The present invention relates to a method and a device for the testing of non-componented circuit boards.
Devices for the testing of non-componented circuit boards may in principle be divided into two groups, namely finger testers and parallel testers. Parallel testers are test devices which, by means of an adapter, simultaneously contact all or at least the majority of contact points of a circuit board to be tested. Finger testers are testers, for the testing of non-componented or componented printed circuit boards, which scan the individual contact points in sequence or serially using two or more test fingers. Serial testing with a finger tester is, due to the principle on which it is based, slower than parallel testing with a parallel tester.
The test fingers are generally attached to a slide which may be moved along a cross-bar, while the cross-bar in turn is guided and able to move on guide rails. The slides may therefore be positioned at any desired point of a generally rectangular test array. Equally there are test devices with fixed cross-bars, on which slides are designed to move. Mounted on these slides are test fingers which are of a certain length and at one end are attached pivotably to the slide. Through the swivelling movement of the test finger, a certain area at right-angles to the cross-bar may be scanned. All circuit board test points of a circuit board to be tested may be contacted and thus tested by both types of finger tester.
A finger tester is described in EP 0 468 153 A1 and a method for the testing of circuit boards using a finger tester is described in EP 0 853 242 A1.
The key factor for the success of a finger tester on the market is the speed with which a circuit board to be tested can be tested. In order to accelerate this test rate, special test methods (e.g. EP 0 853 242 A1 and the corresponding U.S. Pat. No. 5,977,776) or special test probes (e.g. U.S. Pat. No. 5,804,982 and U.S. Pat. No. 6,344,751) have been developed. Such a test probe for the rapid contacting of a circuit board test point of a circuit board to be tested is disclosed by U.S. Pat. No. 5,113,133.
Disclosed by WO 92/11541 is an imaging system for a device for the testing of circuit boards. This imaging system has, like an X-Y recorder, a movable cross-bar on which is mounted a test head with a vertically movable test needle. Mounted next to the test needle is a camera comprising a lens and a CCD element. The image produced by the camera is displayed on a monitor. With the aid of the image shown on the screen, an operator may control the test head in such a way that—during a learning phase—it traces all contact points to be tested and programs the relevant coordinates. During the test the device traces the individual contact points automatically and contacts them with the test needle.
With this known device, therefore, the movement of the contact finger relative to a circuit board is learned with the aid of a camera. With current circuit boards this is practically no longer possible, since the circuit boards have so many circuit board test points, each of which must be contacted by a contact finger, that such a learning process can neither be completed within a reasonably economical period of time, nor can it be made without errors, due to the extent of the number of contacting processes. Consequently, in the case of current finger testers, the test fingers are controlled with the aid of CAD data of the circuit board to be tested. There are various formats for these CAD data, with the most widely used being the so-called Gerber data.
With conventional finger testers, so that the test fingers may be positioned correctly, two calibration processes are performed before the actual testing of the circuit board to be tested. In a first testing process the test heads, each comprised of a slide and a contact finger, are calibrated with reference to the tester, and in a second calibration process, the CAD data of a circuit board to be tested are brought into agreement with an actual circuit board inserted in the tester.
In the first calibration process a calibration plate is inserted in the tester. This calibration plate is a large circuit board on the surface of which a grid is formed by means of the conductor paths. The intersection points of the grid are contacted by the contact fingers. With the aid of these contacts, the respective position of the contact finger may be determined and the tester calibrated.
The second calibration process is generally performed for each circuit board to be tested. In this process, one circuit board of a batch is inserted in the tester and then, using the test fingers or a camera moving over the circuit board, prominent circuit board test points of the pattern of circuit board test points to be tested are detected and their position in the tester determined. When the circuit board test points have been detected, then the CAD data of the circuit board test points of the circuit board to be tested can be brought into agreement with the coordinates of the physical circuit board, i.e. the distortions and misalignments of the circuit board test point pattern typical for a batch are recorded and stored in memory.
When both calibration processes have been completed, then the circuit board test points of the circuit board to be tested may be successfully approached, and contacted by means of the test fingers.
Known from WO 03/096037 is a method in which, during the testing process, the contact tips are monitored by an optical detection unit. Movement of the contact tips is automatically corrected by means of the results determined by the optical detection unit, at least on approaching some of the circuit board test point of a circuit board to be tested, in such a way that the contact tip makes reliable contact with the relevant circuit board test point. In this way circuit board test points are contacted precisely, even when the relevant positioning data has not yet been calibrated or not yet precisely calibrated.
Calibration data may be calculated from the correction data provided by this process. These calibration data replace the two calibration processes usually undertaken with conventional finger testers, since they determine clearly the spatial relationship between the contact tips of the test fingers and the physical circuit board test points of the circuit board to be tested.
Also known, from DE 42 21 075 A1 and the corresponding U.S. Pat. No. 5,596,283 is a method of testing circuit boards in which a finger tester is provided with a device for aligning the circuit board with the measuring system.
This aligning device has a camera for detecting the circuit board. This is used to search for predetermined circuit components and/or structures via conductor paths and connections on the surface of the circuit board. These features and/or components of the circuit board are automatically localised, and an alignment transformation is made between the coordinate system of the circuit board and the coordinate of the movement system.
Using these known systems, therefore, precise positioning of the test fingers relative to the circuit board test points is obtained.
Known from DE 197 03 982 A1 is a method for the testing of circuit boards which is carried out with a finger tester. After placing a circuit board to be tested in the finger tester, the circuit board is optically scanned and the pattern of test points arranged on the circuit board is recorded. Any deviation from an ideal circuit board, ideally placed in the finger tester, is determined and a coordinate system of the finger tester is corrected on the basis of the deviation found. The test fingers of the finger tester are moved in accordance with the corrected coordinate system.
Despite these automatic calibration procedures it is still always necessary to correct manually the data for controlling the test fingers. Such manual correction is required since the circuit boards actually to be tested not only have deviations between the actual position of the individual circuit board test point and their required positions as preset by the CAD data, but also the individual circuit board test points sometimes occur in a different form or a different type (pad field or plated-through hole) from that shown in the CAD data. This is a problem which will continue to exist for at least as long as multi-layer circuit boards are made. For in the production of multi-layer circuit boards, the original CAD data which define the while circuit board are converted into several separate construction drawings for the individual layers. These construction drawings for the individual layers are often produced in a different data format from the CAD data describing the whole circuit board. In the course of data conversion, inaccuracies may occur. In addition, changes are sometimes made by production specialists, in which for example SMD pads are replaced by plated-through holes and vice-versa, in order to simplify the production of a circuit board. As a rule, though, these changes are not entered into the CAD data describing the whole circuit board. Also, for technical production reasons, the pattern used in application of the solder resist to the individual layers is sometimes changed. This may affect the testing of circuit boards in particular if solder resist on the outer surface of a circuit board is changed, since this can change the shapes of the individual circuit board test point, in particular the pad fields. Thus it sometimes happens that rectangular SMD pad fields are changed into circular SMD pad fields, or elongated strip-like pad fields are changed into e.g. rectangular SMD pad fields.
These deviations from the specified CAD data of a specific circuit board type to be tested make it necessary to adjust manually the control data of a finger tester for each type of circuit board and for each batch of circuit boards.
In practice, a test is first of all made without manual adjustment. When problems occur, which is generally the case, the problem points are manually identified and suitably corrected. Until all problem points have been dealt with, this process must be repeated several times. Depending on the complexity of the circuit board type, this may take from several hours up to a whole day. If, instead of pad fields, plated-through holes are formed on a circuit board, it may also happen that a test finger enters this plated-through hole with its contact tip of thin wire, and is torn off in a further sideways movement of the test finger. This then leads to a failure of the entire tester, generating additional material costs through replacement of the test probe.